Noncontact measurement of the temperature (assumed to be substantially uniform) of a thermally radiating body at elevated temperature is difficult, for a number of reasons. First, the body is usually a "greybody", and thus has an associated emissivity parameter .beta. (also known) that is less than 1.0. A thermal "blackbody" would have an emissivity parameter .beta. that is =1.0 for all wavelengths. Second, the emissivity varies with the wavelength at which the thermal radiation is measured. Third, the perceived temperature and emissivity will be affected by any intervening medium, such as air or other low pressure gas, that the body is positioned in. This third difficulty may be substantially avoided by conducting the measurements in a high vacuum environment.
Recently, some workers have published discussions of remote sensing of temperature, not necessarily uniform, of a radiating body and possible discrimination between radiation emitted by the body and radiation emitted by other objects adjacent to the body. For example, Cogan, in "Remote Sensing of Surface and Near Surface Temperature from Remotely Piloted Aircraft," Applied Optics 24 (1985) 1030-1036, discusses a technique for obtaining the temperature of specific water and land surfaces that are adjacent to other radiating bodies. Cogan utilizes a normalized radiance, which is the product of target radiance and the response curve function for the measuring device, both wavelength dependent, integrated over a predetermined wavelength range and divided by the integral of the response curve function over the same wavelength range. Cogan also utilizes a mean wavelength, weighted by the response curve function, in the well known Planck formula, Equation (2) below, to determine a measured mean atmospheric temperature. However, Cogan does not take separate account of the wavelength-dependent emissivity or decompose the radiation into a sequence of wavelength ranges for purposes of separate determination of the true temperature of the radiating body and the true body emissivity in a given wavelength range.
Rockstroh and Mazumder, in "Infrared Thermographic Temperature Measurement During Laser Heat Treatment," Applied Optics, 24 (1985) 1343-1345, simultaneously use a thermocouple to measure ambient temperature and a thermographic system to measure the greybody temperature of an adjacent body; and they attempt to predict local surface temperature of the greybody, assuming a constant emissivity of 0.8. Wavelength dependence of emissivity is not addressed, and no separate account is taken of the wavelength dependence of the radiance itself.
The apparent temperatures of actively burning regions and burned over regions in a forest fire are assessed by Stearns et al. in "Airborne Infrared Observations and Analyses of a Large Forest Fire," Applied Optics (1986) 2554-2562. Thermal scanners are used with small fields of view to determine apparent temperature of different regions in and around an active forest fire. The spectral-radiance of a forest fire is obtained as a function of wave number (cm.sup.-1) at a quoted spectral-resolution of 0.95 cm.sup.-1. The method used, if any, for accounting for the wavelength dependence of emissivity is not disclosed in the Stearns et al. paper, although a method of indirectly determining apparent temperature of local areas adjacent to a forest fire is apparently employed.
In "Quantative Measurements of Ambient Radiation, Emissivity, and Truth Temperature of a Greybody: Methods and Experimental Results," Applied Optics (1986) 3683-3689, Zhang, Zhang and Klemas discuss the contribution to radiance from self-radiation of a target and from ambient radiation reflected by the target. This paper notes the central importance of the emissivity and the truth temperature of a greybody, and it formally utilizes the Planck formula given in Equation (2) below, to formally manipulate certain integral expressions for body radiance in the presence of reflected radiation that arises from adjacent objects. The paper discusses the difficulty of distinguishing between temperatures of two bodies with similar but not identical emissivities and presents a method for obtaining the approximate emissivity of a greybody, where the body temperature can be both measured accurately and increased or decreased by a controllable amount. However, the possible wavelength dependence of emissivity is not separately taken account of in this paper.
The subject invention provides method and apparatus for noncontact determination of the temperature and the emissivity, within a predetermined narrow wavelength range, of a radiating body.